• N SIVANANTHAM

Articles written in Bulletin of Materials Science

• Effect of heat treatment on the optical properties of layered muscovite single crystal sheets

Naturally, thin sheets of layered, dioctahedral muscovite single crystals are transparent to visible light. Upon heat treatments performed up to 900$^{\circ}$C, the single crystal sheets of thickness $\sim$200 $\mu$m exhibited changes in its optical transparent character. The muscovite sheets exhibited either translucent or nontransparent, dark-brown, gold or copper like luster, varying optical band gap (from $\sim$3.7 to 2.7 eV) and photoluminescence (PL) properties, which depend on the temperature of the heat treatment. A comprehensive investigation has been made using X-ray diffraction, thermal analyses, energy dispersive X-ray and Raman spectroscopies to understand how/why the optical properties of the muscovite sheets changed up on heat treatments. Besides, no phase changes were found in the layered, lamellar crystal structure, evidences for the known dehydroxylation of muscovite were clearly seen from the thermal analyses data. Further, the Raman spectrum of the muscovite sheet heat treated at 900$^{\circ}$C showed no evidence for the presence of hydroxyl group (OH$^−$), which confirmed the completion of the dehydroxylation. The dihydroxylation-induced changes in the structural-elements (for example, Al-(O,OH)$_6$ partly or fully changing to Al-O$_5$/O$_6$ and its effects on the lattice constants, and also the defect levels introduced by the heat treatments are responsible for the changes in the optical transparency, colour of appearance, band gap and PL of the muscovite natural single crystal sheets.

• Ferromagnetism in Gd-doped ZnO thin films mediated by defects

Defects play an inevitable role in controlling the optical and magnetic properties of ZnO. In this study, defects were introduced in gadolinium (Gd)-doped ZnO films by depositing in pure argon atmosphere. The pristine- and Gddoped (0.05, 0.1 and 1 at%) films were deposited on Si(111) substrate by radio frequency magnetron sputtering at a substrate temperature of 450°C under Ar pressure of 0.02 mbar. Structural, morphological, chemical, optical and magnetic properties of the deposited films were studied by X-ray diffraction and Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, photoluminescence and vibrating sample magnetometer, respectively. It is confirmed that oxygen-deficient growth condition leads to the formation of oxygen vacancy (V$_O$$^+) and zinc interstitial (Zn_i$$^+$) defects in the films. It is shown that a critical amount of Zn$_i$$^+ and V_O$$^+$ along with the appropriate amount of Gd$^{3+}$ ions are required to induce room temperature ferromagnetism in Gd-doped ZnO thin film deposited on Si(111) substrate. A possible mechanism has been proposed based on bound magnetic polaron model to explain the observed ferromagnetism.

• # Bulletin of Materials Science

Volume 45, 2022
All articles
Continuous Article Publishing mode

• # Dr Shanti Swarup Bhatnagar for Science and Technology

Posted on October 12, 2020

Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
Chemical Sciences 2020